Water soluble diphosphine ligands based on 1,3,5-triaza-7-phosphaadamantane (PTA-PR2): Synthesis, coordination chemistry, and ruthenium catalyzed nitrile hydration

The Backbone of Success of P,N-Hybrid Ligands: Some Recent Developments

Organophosphorus ligands are an invaluable family of compounds that continue to underpin important roles in disciplines such as coordination chemistry and catalysis. Their success can routinely be traced back to facile tuneability thus enabling a high degree of control over, for example, electronic and steric properties. Diphosphines, phosphorus compounds bearing two separated P^III donor atoms, are also highly valued and impart their own unique features, for example excellent chelating properties upon metal complexation. In many classical ligands of this type, the backbone connectivity has been based on all carbon spacers only but there is growing interest in embedding other donor atoms such as additional nitrogen (–NH–, –NR–) sites. This review will collate some important examples of ligands in this field, illustrate their role as ligands in coordination chemistry and highlight some of their reactivities and applications. It will be shown that incorporation of a nitrogen-based group can impart unusual reactivities and important catalytic applications.

Enzymatic Approach to the Synthesis of Enantiomerically Pure Hydroxy Derivatives of 1,3,5-Triaza-7-phosphaadamantane

A series of enantiomerically pure derivatives of 6-(1-hydroxyalkyl)-1,3,5-triaza-7-phosphatricyclo[3.3.1.1]decane 5 were successfully synthesized for the first time. A series of hydrolytic enzymes was applied in a stereoselective acetylation performed under kinetic resolution conditions. Although the secondary alcohols: α-aryl-hydroxymethyl-PTA (phosphines) 5b-d', PTA-oxides 8b-d', and PTA-sulfides 9b-d' were found to be totally unreactive in the presence of all the enzymes and various conditions applied, the primary alcohols, i.e., the hydroxymethyl derivatives PTA oxide 8a and PTA sulfide 9a, were successfully resolved into enantiomers with moderate to good enantioselectivity (up to 95%). The absolute configurations of the products were determined by an X-ray analysis and chemical correlation.

Hydration of Aliphatic Nitriles Catalyzed by an Osmium Polyhydride: Evidence for an Alternative Mechanism

The hexahydride OsH₆(PⁱPr₃)₂ competently catalyzes the hydration of aliphatic nitriles to amides. The main metal species under the catalytic conditions are the trihydride osmium(IV) amidate derivatives OsH₃{κ²-N,O-[HNC(O)R]}(PⁱPr₃)₂, which have been isolated and fully characterized for R = ⁱPr and ^tBu. The rate of hydration is proportional to the concentrations of the catalyst precursor, nitrile, and water. When these experimental findings and density functional theory calculations are combined, the mechanism of catalysis has been established. Complexes OsH₃{κ²-N,O-[HNC(O)R]}(PⁱPr₃)₂ dissociate the carbonyl group of the chelate to afford κ¹-N-amidate derivatives, which coordinate the nitrile. The subsequent attack of an external water molecule to both the C(sp) atom of the nitrile and the N atom of the amidate affords the amide and regenerates the κ¹-N-amidate catalysts. The attack is concerted and takes place through a cyclic six-membered transition state, which involves C_nitrile···O–H···N_amidate interactions. Before the attack, the free carbonyl group of the κ¹-N-amidate ligand fixes the water molecule in the vicinity of the C(sp) atom of the nitrile.

The hexahydride complex OsH₆(PⁱPr₃)₂ competently catalyzes the hydration of aliphatic nitriles to amides. Isolation of the main metal species under the catalytic conditions, kinetics of hydration, and density functional theory calculations support an alternative mechanism to those previously reported.

Synthesis of Hydrophilic Phosphorus Ligands and Their Application in Aqueous-Phase Metal-Catalyzed Reactions

Transition metal-catalyzed reactions in aqueous media are experiencing a constant increase in interest. In homogenous catalysis the use of water as a solvent offers advantages in cost, safety, the possibility of two-phase catalysis and simplified separation strategies. In the life sciences, transition metal catalysis in aqueous systems enables the ligation or modification of biopolymers in buffer systems or even in their cellular environment. In biocatalysis, aqueous systems allow the simultaneous use of enzymes and transition metal catalysts in cascade reactions. The use of water-soluble phosphine ligands still represents the most reliable and popular strategy for transferring metal catalysts into the aqueous phase. This review summarizes the recent advancements in this field since 2009 and describes current synthetic strategies for the preparation of hydrophilic phosphines and phosphites. In addition, recent applications of transition metal catalysis in aqueous solvents using these hydrophilic ligands are presented.

Crystal structures of trans-acetyl-dicarbon-yl(η5-cyclo-penta-dien-yl)(1,3,5-tri-aza-7-phosphaadamantane)molybdenum(II) and trans-acetyl-di-carbon-yl(η5-cyclo-penta-dien-yl)(3,7-diacetyl-1,3,7-tri-aza-5-phosphabi-cyclo-[3.3.1]nona-ne)molyb-den-um(II)

The title compounds, [Mo(C5H5)(COCH3)(C6H12N3P)(CO)2], (1), and [Mo(C5H5)(COCH3)(C9H16N3O2P)(C6H5)2))(CO)2], (2), have been prepared by phosphine-induced migratory insertion from [Mo(C5H5)(CO)3(CH3)]. The mol-ecular structures of these complexes are quite similar, exhibiting a four-legged piano-stool geometry with trans-disposed carbonyl ligands. The extended structures of complexes (1) and (2) differ substanti-ally. For complex (1), the molybdenum acetyl unit plays a dominant role in the organization of the extended structure, joining the mol-ecules into centrosymmetrical dimers through C-H⋯O inter-actions with a cyclo-penta-dienyl ligand of a neighboring mol-ecule, and these dimers are linked into layers parallel to (100) by C-H⋯O inter-actions between the molybdenum acetyl and the cyclo-penta-dienyl ligand of another neighbor. The extended structure of (2) is dominated by C-H⋯O inter-actions involving the carbonyl groups of the acetamide groups of the DAPTA ligand, which join the mol-ecules into centrosymmetrical dimers and link them into chains along [010]. Additional C-H⋯O inter-actions between the molybdenum acetyl oxygen atom and an acetamide methyl group join the chains into layers parallel to (101).

Pentafluorophenyl Platinum(II) Complexes of PTA and its N-Allyl and N-Benzyl Derivatives: Synthesis, Characterization and Biological Activity

From the well-known 1,3,5-triaza-phosphaadamantane (PTA, 1a), the novel N-allyl and N-benzyl tetrafuoroborate salts 1-allyl-1-azonia-3,5-diaza-7-phosphaadamantane (APTA(BF₄), 1b) and 1-benzyl-1-azonia-3,5-diaza-7-phosphaadamantane (BzPTA(BF₄), 1c) were obtained. These phosphines were then allowed to react with (Pt(μ-Cl)(C₆F₅)(tht))₂ (tht = tetrahydrothiophene) affording the water soluble Pt(II) complexes trans-(PtCl(C₆F₅)(PTA)₂) (2a) and its bis-cationic congeners trans-(PtCl(C₆F₅)(APTA)₂)(BF₄)₂ (2b) and trans-(PtCl(C₆F₅)(BzPTA)₂)(BF₄)₂ (2c). The compounds were fully characterized by multinuclear NMR, ESI-MS, elemental analysis and (for 2a) also by single crystal X-ray diffraction, which proved the trans configuration of the phosphine ligands. Furthermore, in order to evaluate the cytotoxic activities of all complexes the normal human dermal fibroblast (NHDF) cell culture were used. The antineoplastic activity of the investigated compounds was checked against the human lung carcinoma (A549), epithelioid cervix carcinoma (HeLa) and breast adenocarcinoma (MCF-7) cell cultures. Interactions between the complexes and human serum albumin (HSA) using fluorescence spectroscopy and circular dichroism spectroscopy (CD) were also investigated.

Unusual Water-Soluble Imino Phosphine Ligand: Enamine and Imine Derivatives of 1,3,5-Triaza-7-phosphaadamantane (PTA)

A series of water-soluble and air-stable E-enamine derivatives of 1,3,5-triaza-7-phosphaadamantane (PTA), PTA═C(R)NH₂, 1-4, are reported along with data on E-Z isomerization and tautomerization to the imine form (PTA-CR═NH). Reaction of 1,3,5-triaza-7-phosphaamantane-6-yl lithium, PTA-Li, with aromatic nitriles afforded E-enamine derivatives of PTA in good yield (49-91%). Phosphines 1-4 are stable toward water and air, and do not appear to isomerize or tautomerize, unless coordinated to a metal or oxidized. The corresponding oxides, O═PTA═C(R)NH₂ (5-8), were observed as ∼55/45 mixtures of E and Z isomers. Kinetic data on the E-Z isomerization is reported. Upon coordination of 1-4 to W(CO)₄(pip)₂, a κ¹- P enamine is formed, [W(CO)₄(pip)(κ¹- P-PTA═CRNH₂)]. Enamine-imine tautomerization of the metal bound phosphine was observed resulting in κ²- P, N imine complexes, [W(CO)₄(κ²- P, N-PTA-CR═NH)], 9-12. The crystal structures of the κ¹- P enamine 11a, κ²- P, N imine 9 and 12, phosphines 1 and 3, as well as phosphine oxide 8a were obtained. DFT calculations on the various isomers of the phosphines and phosphine oxides are also reported.